Involvement of the choroid plexus in immune complex disease is demonstrated in experimental diseases, i.e. acute serum sickness of rabbits, Ferritin and nuclear antigen induced chronic immune complex disease of mice. The animals are injected intravenously with horseradish peroxidase or Thorotrast for the purpose of demonstrating leakage of tracers into the spinal fluid associated with the deposition of immune complexes. Human choroid plexus and renal cortex from consecutive autopsies are screened for deposits of IgG, IgM, C3 and albumin as well as for the binding of sheep red cells coated with C3b. The methods are electron microscopy and immunofluorescence. BIBLIOGRAPHIC REFERENCES: Lampert, P.W., Joseph, B.S., Oldstone, M.B.A.: Morphologic Changes of Cells Infected with Measles or Related Viruses, In Progress in Neurophathology, Ed. Zimmerman, H.M. Grune and Stratton, New York, 1976, 55-68. Powell, H., Knox, D., Lee, S., Charters, A.C., Orloff, M., Garrett, R., Lampert, P.; Alloxan Diabetic Neuropathy, Electron Microscopic Studies, Neurology, 27: 60-66, 1977.